Interpretive Summary: Sub-toxic levels of herbicides sometimes stimulate growth of plants, a phenomenon called hormesis. Hormesis has been reported with the world’s most used herbicide, glyphosate. This work demonstrates for the first time that glyphosate-stimulated growth of coffee plants is dependent on the developmental stage of the plant. This finding suggests that when glyphosate is used to kill weeds in coffee plantations, drift from the spray may stimulate growth of the crop at certain growth stages.

Technical Abstract:
Weed management systems in almost all Brazilian coffee plantations allow herbicide spray to drift on crop plants. In order to evaluate if there is any effect of the most commonly used herbicide in coffee production, glyphosate, on coffee plants, a range of glyphosate doses were applied directly on coffee plants at two distinct plant growth stages. Although growth of both young and old plants was reduced at higher glyphosate doses, low doses caused no effects on growth characteristics of young plants and stimulated growth of older plants. Therefore, hormesis with glyphosate is dependent on coffee plant growth stage at the time of herbicide application.